A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is desirable to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
An important consideration when creating beams for metrology measurement is the homogeneity of the beam. Optical systems such as lenses that have been used to focus the radiation beam have not generally contributed to the collimation of the beam; the output direction of travel of the radiation beam is dependent on the input direction of the beam, which may not have been homogenized in the first place.
In scatterometry, misalignments and overlay errors of properties of a substrate are determined by the reflected spectra of a radiation beam having been reflected from the substrate in question. In order that the reflected (and scattered) spectrum is a true representation of the surface of the substrate, it is desirable that the properties of the beam before it is reflected are also known. As measuring the properties of a beam interferes with it, it is desirable to make the beam to precise standards in the first place, with occasional measurements being possible to ensure that the properties of the beam have not changed. The most desirable property of an incident beam is that it is homogenous. Any inhomogeneity in the beam before it is reflected may contribute to inconsistencies in the reflected beam, which may be interpreted as effects caused by the surface of the substrate, rather than by inhomogeneities of the beam. This may lead to errors in the measurement of the surface of the substrate.